# Interoperability#

CuPy can be used in conjunction with other libraries.

## CUDA functionalities#

### Device management#

Under construction. Starting CuPy v10, the `with Device`

context manager would no longer respect `cudaSetDevice()`

, see Change in cupy.cuda.Device Behavior.

### Stream and event management#

Under construction. For using CUDA streams created in foreign libraries in CuPy, see Streams and Events.

## NumPy#

`cupy.ndarray`

implements `__array_ufunc__`

interface (see NEP 13 — A Mechanism for Overriding Ufuncs for details).
This enables NumPy ufuncs to be directly operated on CuPy arrays.
`__array_ufunc__`

feature requires NumPy 1.13 or later.

```
import cupy
import numpy
arr = cupy.random.randn(1, 2, 3, 4).astype(cupy.float32)
result = numpy.sum(arr)
print(type(result)) # => <class 'cupy._core.core.ndarray'>
```

`cupy.ndarray`

also implements `__array_function__`

interface (see NEP 18 — A dispatch mechanism for NumPy’s high level array functions for details).
This enables code using NumPy to be directly operated on CuPy arrays.
`__array_function__`

feature requires NumPy 1.16 or later; As of NumPy 1.17, `__array_function__`

is enabled by default.

## Numba#

Numba is a Python JIT compiler with NumPy support.

`cupy.ndarray`

implements `__cuda_array_interface__`

, which is the CUDA array interchange interface compatible with Numba v0.39.0 or later (see CUDA Array Interface for details).
It means you can pass CuPy arrays to kernels JITed with Numba.
The following is a simple example code borrowed from numba/numba#2860:

```
import cupy
from numba import cuda
@cuda.jit
def add(x, y, out):
start = cuda.grid(1)
stride = cuda.gridsize(1)
for i in range(start, x.shape[0], stride):
out[i] = x[i] + y[i]
a = cupy.arange(10)
b = a * 2
out = cupy.zeros_like(a)
print(out) # => [0 0 0 0 0 0 0 0 0 0]
add[1, 32](a, b, out)
print(out) # => [ 0 3 6 9 12 15 18 21 24 27]
```

In addition, `cupy.asarray()`

supports zero-copy conversion from Numba CUDA array to CuPy array.

```
import numpy
import numba
import cupy
x = numpy.arange(10) # type: numpy.ndarray
x_numba = numba.cuda.to_device(x) # type: numba.cuda.cudadrv.devicearray.DeviceNDArray
x_cupy = cupy.asarray(x_numba) # type: cupy.ndarray
```

Warning

`__cuda_array_interface__`

specifies that the object lifetime must be managed by the user, so it is an undefined behavior if the
exported object is destroyed while still in use by the consumer library.

Note

CuPy uses two environment variables controlling the exchange behavior: `CUPY_CUDA_ARRAY_INTERFACE_SYNC`

and `CUPY_CUDA_ARRAY_INTERFACE_EXPORT_VERSION`

.

## mpi4py#

MPI for Python (mpi4py) is a Python wrapper for the Message Passing Interface (MPI) libraries.

MPI is the most widely used standard for high-performance inter-process communications. Recently several MPI vendors, including MPICH, Open MPI and MVAPICH, have extended their support beyond the MPI-3.1 standard to enable “CUDA-awareness”; that is, passing CUDA device pointers directly to MPI calls to avoid explicit data movement between the host and the device.

With the `__cuda_array_interface__`

(as mentioned above) and `DLPack`

data exchange protocols (see DLPack below) implemented in CuPy, mpi4py now provides (experimental) support for passing CuPy arrays to MPI calls, provided that mpi4py is built against a CUDA-aware MPI implementation. The following is a simple example code borrowed from mpi4py Tutorial:

```
# To run this script with N MPI processes, do
# mpiexec -n N python this_script.py
import cupy
from mpi4py import MPI
comm = MPI.COMM_WORLD
size = comm.Get_size()
# Allreduce
sendbuf = cupy.arange(10, dtype='i')
recvbuf = cupy.empty_like(sendbuf)
comm.Allreduce(sendbuf, recvbuf)
assert cupy.allclose(recvbuf, sendbuf*size)
```

This new feature is added since mpi4py 3.1.0. See the mpi4py website for more information.

## PyTorch#

PyTorch is a machine learning framefork that provides high-performance, differentiable tensor operations.

PyTorch also supports `__cuda_array_interface__`

, so zero-copy data exchange between CuPy and PyTorch can be achieved at no cost.
The only caveat is PyTorch by default creates CPU tensors, which do not have the `__cuda_array_interface__`

property defined, and
users need to ensure the tensor is already on GPU before exchanging.

```
>>> import cupy as cp
>>> import torch
>>>
>>> # convert a torch tensor to a cupy array
>>> a = torch.rand((4, 4), device='cuda')
>>> b = cp.asarray(a)
>>> b *= b
>>> b
array([[0.8215962 , 0.82399917, 0.65607935, 0.30354425],
[0.422695 , 0.8367199 , 0.00208597, 0.18545236],
[0.00226746, 0.46201342, 0.6833052 , 0.47549972],
[0.5208748 , 0.6059282 , 0.1909013 , 0.5148635 ]], dtype=float32)
>>> a
tensor([[0.8216, 0.8240, 0.6561, 0.3035],
[0.4227, 0.8367, 0.0021, 0.1855],
[0.0023, 0.4620, 0.6833, 0.4755],
[0.5209, 0.6059, 0.1909, 0.5149]], device='cuda:0')
>>> # check the underlying memory pointer is the same
>>> assert a.__cuda_array_interface__['data'][0] == b.__cuda_array_interface__['data'][0]
>>>
>>> # convert a cupy array to a torch tensor
>>> a = cp.arange(10)
>>> b = torch.as_tensor(a, device='cuda')
>>> b += 3
>>> b
tensor([ 3, 4, 5, 6, 7, 8, 9, 10, 11, 12], device='cuda:0')
>>> a
array([ 3, 4, 5, 6, 7, 8, 9, 10, 11, 12])
>>> assert a.__cuda_array_interface__['data'][0] == b.__cuda_array_interface__['data'][0]
```

PyTorch also supports zero-copy data exchange through `DLPack`

(see DLPack below):

```
import cupy
import torch
from torch.utils.dlpack import to_dlpack
from torch.utils.dlpack import from_dlpack
# Create a PyTorch tensor.
tx1 = torch.randn(1, 2, 3, 4).cuda()
# Convert it into a DLPack tensor.
dx = to_dlpack(tx1)
# Convert it into a CuPy array.
cx = cupy.from_dlpack(dx)
# Convert it back to a PyTorch tensor.
tx2 = from_dlpack(cx.toDlpack())
```

pytorch-pfn-extras library provides additional integration features with PyTorch, including memory pool sharing and stream sharing:

```
>>> import cupy
>>> import torch
>>> import pytorch_pfn_extras as ppe
>>>
>>> # Perform CuPy memory allocation using the PyTorch memory pool.
>>> ppe.cuda.use_torch_mempool_in_cupy()
>>> torch.cuda.memory_allocated()
0
>>> arr = cupy.arange(10)
>>> torch.cuda.memory_allocated()
512
>>>
>>> # Change the default stream in PyTorch and CuPy:
>>> stream = torch.cuda.Stream()
>>> with ppe.cuda.stream(stream):
... ...
```

### Using custom kernels in PyTorch#

With the DLPack protocol, it becomes very simple to implement functions in PyTorch using CuPy user-defined kernels. Below is the example of a PyTorch autograd function
that computes the forward and backward pass of the logarithm using `cupy.RawKernel`

s.

```
import cupy
import torch
cupy_custom_kernel_fwd = cupy.RawKernel(
r"""
extern "C" __global__
void cupy_custom_kernel_fwd(const float* x, float* y, int size) {
int tid = blockDim.x * blockIdx.x + threadIdx.x;
if (tid < size)
y[tid] = log(x[tid]);
}
""",
"cupy_custom_kernel_fwd",
)
cupy_custom_kernel_bwd = cupy.RawKernel(
r"""
extern "C" __global__
void cupy_custom_kernel_bwd(const float* x, float* gy, float* gx, int size) {
int tid = blockDim.x * blockIdx.x + threadIdx.x;
if (tid < size)
gx[tid] = gy[tid] / x[tid];
}
""",
"cupy_custom_kernel_bwd",
)
class CuPyLog(torch.autograd.Function):
@staticmethod
def forward(ctx, x):
ctx.input = x
# Enforce contiguous arrays to simplify RawKernel indexing.
cupy_x = cupy.ascontiguousarray(cupy.from_dlpack(x.detach()))
cupy_y = cupy.empty(cupy_x.shape, dtype=cupy_x.dtype)
x_size = cupy_x.size
bs = 128
cupy_custom_kernel_fwd(
(bs,), ((x_size + bs - 1) // bs,), (cupy_x, cupy_y, x_size)
)
# the ownership of the device memory backing cupy_y is implicitly
# transferred to torch_y, so this operation is safe even after
# going out of scope of this function.
torch_y = torch.from_dlpack(cupy_y)
return torch_y
@staticmethod
def backward(ctx, grad_y):
# Enforce contiguous arrays to simplify RawKernel indexing.
cupy_input = cupy.from_dlpack(ctx.input.detach()).ravel()
cupy_grad_y = cupy.from_dlpack(grad_y.detach()).ravel()
cupy_grad_x = cupy.zeros(cupy_grad_y.shape, dtype=cupy_grad_y.dtype)
gy_size = cupy_grad_y.size
bs = 128
cupy_custom_kernel_bwd(
(bs,),
((gy_size + bs - 1) // bs,),
(cupy_input, cupy_grad_y, cupy_grad_x, gy_size),
)
# the ownership of the device memory backing cupy_grad_x is implicitly
# transferred to torch_y, so this operation is safe even after
# going out of scope of this function.
torch_grad_x = torch.from_dlpack(cupy_grad_x)
return torch_grad_x
```

Note

Directly feeding a `torch.Tensor`

to `cupy.from_dlpack()`

is only supported in the (new) DLPack data exchange protocol added in CuPy v10+ and PyTorch 1.10+.
For earlier versions, you will need to wrap the `Tensor`

with `torch.utils.dlpack.to_dlpack()`

as shown in the above examples.

## RMM#

RMM (RAPIDS Memory Manager) provides highly configurable memory allocators.

RMM provides an interface to allow CuPy to allocate memory from the RMM memory pool instead of from CuPy’s own pool. It can be set up as simple as:

```
import cupy
import rmm
cupy.cuda.set_allocator(rmm.rmm_cupy_allocator)
```

Sometimes, a more performant allocator may be desirable. RMM provides an option to switch the allocator:

```
import cupy
import rmm
rmm.reinitialize(pool_allocator=True) # can also set init pool size etc here
cupy.cuda.set_allocator(rmm.rmm_cupy_allocator)
```

For more information on CuPy’s memory management, see Memory Management.

## DLPack#

DLPack is a specification of tensor structure to share tensors among frameworks.

CuPy supports importing from and exporting to DLPack data structure (`cupy.from_dlpack()`

and `cupy.ndarray.toDlpack()`

).

Here is a simple example:

```
import cupy
# Create a CuPy array.
cx1 = cupy.random.randn(1, 2, 3, 4).astype(cupy.float32)
# Convert it into a DLPack tensor.
dx = cx1.toDlpack()
# Convert it back to a CuPy array.
cx2 = cupy.from_dlpack(dx)
```

TensorFlow also supports DLpack, so zero-copy data exchange between CuPy and TensorFlow through DLPack is possible:

```
>>> import tensorflow as tf
>>> import cupy as cp
>>>
>>> # convert a TF tensor to a cupy array
>>> with tf.device('/GPU:0'):
... a = tf.random.uniform((10,))
...
>>> a
<tf.Tensor: shape=(10,), dtype=float32, numpy=
array([0.9672388 , 0.57568085, 0.53163004, 0.6536236 , 0.20479882,
0.84908986, 0.5852566 , 0.30355775, 0.1733712 , 0.9177849 ],
dtype=float32)>
>>> a.device
'/job:localhost/replica:0/task:0/device:GPU:0'
>>> cap = tf.experimental.dlpack.to_dlpack(a)
>>> b = cp.from_dlpack(cap)
>>> b *= 3
>>> b
array([1.4949363 , 0.60699713, 1.3276931 , 1.5781245 , 1.1914308 ,
2.3180873 , 1.9560868 , 1.3932796 , 1.9299742 , 2.5352407 ],
dtype=float32)
>>> a
<tf.Tensor: shape=(10,), dtype=float32, numpy=
array([1.4949363 , 0.60699713, 1.3276931 , 1.5781245 , 1.1914308 ,
2.3180873 , 1.9560868 , 1.3932796 , 1.9299742 , 2.5352407 ],
dtype=float32)>
>>>
>>> # convert a cupy array to a TF tensor
>>> a = cp.arange(10)
>>> cap = a.toDlpack()
>>> b = tf.experimental.dlpack.from_dlpack(cap)
>>> b.device
'/job:localhost/replica:0/task:0/device:GPU:0'
>>> b
<tf.Tensor: shape=(10,), dtype=int64, numpy=array([0, 1, 2, 3, 4, 5, 6, 7, 8, 9])>
>>> a
array([0, 1, 2, 3, 4, 5, 6, 7, 8, 9])
```

Be aware that in TensorFlow all tensors are immutable, so in the latter case any changes in `b`

cannot be reflected in the CuPy array `a`

.

Note that as of DLPack v0.5 for correctness the above approach (implicitly) requires users to ensure that such conversion (both importing and exporting a CuPy array) must happen on the same CUDA/HIP stream. If in doubt, the current CuPy stream in use can be fetched by, for example, calling `cupy.cuda.get_current_stream()`

. Please consult the other framework’s documentation for how to access and control the streams.

### DLPack data exchange protocol#

To obviate user-managed streams and DLPack tensor objects, the DLPack data exchange protocol provides a mechanism to shift the responsibility from users to libraries. Any compliant objects (such as `cupy.ndarray`

) must implement a pair of methods `__dlpack__`

and `__dlpack_device__`

. The function `cupy.from_dlpack()`

accepts such object and returns a `cupy.ndarray`

that is safely accessible on CuPy’s current stream. Likewise, `cupy.ndarray`

can be exported via any compliant library’s `from_dlpack()`

function.

Note

CuPy uses `CUPY_DLPACK_EXPORT_VERSION`

to control how to handle tensors backed by CUDA managed memory.